TestUniswapVy.vy

# @version ^0.2
from vyper.interfaces import ERC20

# interface UniswapV2Router:
#   def swapExactTokensForTokens(amountIn: uint256, amountOutMin: uint256, path: address[3], to: address, deadline: uint256): nonpayable

UNISWAP_V2_ROUTER: constant(address) = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D
WETH: constant(address) = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2

@external
def swap(tokenIn: address, tokenOut: address, amountIn: uint256):
  ERC20(tokenIn).transferFrom(msg.sender, self, amountIn)
  ERC20(tokenIn).approve(UNISWAP_V2_ROUTER, amountIn)

  # path: address[3] = [ZERO_ADDRESS, ZERO_ADDRESS, ZERO_ADDRESS]
  # if tokenIn == WETH or tokenOut == WETH:
  #   path[0] = tokenIn
  #   path[1] = tokenOut
  # else:
  #   path[0] = tokenIn
  #   path[1] = WETH
  #   path[2] = tokenOut

  # UniswapV2Router(UNISWAP_V2_ROUTER).swapExactTokensForTokens(amount, 0, path, self, block.timestamp + 1800)

  res: Bytes[128] = raw_call(
    UNISWAP_V2_ROUTER,
    concat(
      method_id("swapExactTokensForTokens(uint256,uint256,address[],address,uint256)"),
      convert(amountIn, bytes32),        # amount in
      convert(0, bytes32),               # amount out min
      convert(160, bytes32),             # path[] offset (5 * 32, 5 = number of func args)
      convert(self, bytes32),            # to
      convert(block.timestamp, bytes32), # deadline
      convert(3, bytes32),               # path[] length
      convert(tokenIn, bytes32),
      convert(WETH, bytes32),
      convert(tokenOut, bytes32)
    ),
    max_outsize=128
  )